The invention relates to a sensor, especially a pressure sensor.
By way of example, pressure sensors are known in which thin-film resistor measurement bridges for measuring absolute pressures or pressure changes, particularly in hydraulic systems, are disposed on a measurement diaphragm. Motions of the measurement diaphragm from pressure fluctuations lead to changes of resistance, because of compressive offsets or elongations of what as a rule are meandering resistor tracks, in the various thin-film resistors. The thin-film resistors are connected in a known manner to form a Wheatstone measurement bridge; the association of the thin-film resistors with the bridge branches or the regions on the pressure sensor diaphragm is selected such that the opposed resistors each vary in the same direction, and a bridge diagonal voltage can be measured as a sensor signal.
In the most frequent instances of applications of pressure sensors, such as in hydraulic brake systems in motor vehicles, an accurate output signal corresponding to the pressure of the brake hydraulics (measurement range approximately 250 bar) must be generatable highly reliably and in addition in as fail-safe a way as possible. Especially in systems critical to safety in the area of brake systems, such as the anti-lock system or traction control system, sensors are required, the perfect function of which must also be monitorable continuously. Other applications include monitoring functions in pneumatic systems and in injection systems for delivering fuel in motor vehicles.
It is also known that the monitoring of pressure sensors that have resistor measurement bridges is done in such a way that at specified time intervals an absolute measurement of the individual resistors is done, in order to detect changes, caused for instance by aging or destruction (for instance from corrosion or breakage) of the resistor properties of the individual thin-film resistors. Plastic deformations of the pressure measuring diaphragm from overpressure or tearing of the most severely strained point in the middle of the diaphragm cause incorrect measurements. A change in resistors that vary in the same direction in the bridge branches cannot be detected, without the special provisions already mentioned, since these changes compensate for one another by offset in the measurement bridge, and thus while the measurement bridge appears unchanged from outside, nevertheless its sensitivity changes and thus mistakes.
The resistors of the measurement bridge that each change in the same direction are located on the pressure measuring diaphragm preferentially at locations having the same mechanical properties with respect to tensile elongation or compressive offset (either in the middle or on the edge of the pressure measuring diaphragm) and are therefore under equal strain; their deviations behave accordingly. Plastic deformations of the pressure measuring diaphragm thus also exhibit the same undetectable signal errors. Another known possibility of detecting such errors is to repeat the comparison of the individual resistors with a stable reference resistor at certain intervals. The reference resistor, which is stable over its entire life, can be connected parallel to a bridge resistor for this purpose and therefore used for monitoring changes in the bridge output signal.
The known sensors with the special monitoring mechanisms discussed have the disadvantage above all that a constant switchover is necessary between the test/monitoring mode and pressure sensing, which greatly reduces the dynamics of the sensor since the reference measurement takes time. Moreover, demands for failsafeness and redundancy cannot be met by these provisions.